The field of the present invention is semiconductor switching devices activated by radiant energy, including electromagnetic waves and sound waves. Most prior devices require either an enormous amount of radiant energy or else a second triggering energy source to initiate conduction. There are other prior devices which require only a small amount of radiant energy, and do not need a second triggering energy source, but these devices are subject to unwanted nuisance tripping induced by dv/dt, noise, temperature, etc. There are still other devices which exhibit good turn-on sensitivity but require a second energy source to de-sensitize the device in order to provide immunity to dv/dt.
Generally, semiconductor swithces, such as SCR's, triacs, transistors, etc., can be turned ON to a conductive state by a triggering signal applied to a gate region thereof, whereby load current flows therethrough. In most applications, it is desirable to reduce the gate sensitivity in order to improve immunity to unwanted dv/dt, temperature, etc. induced turn-on. In an SCR for example, it is common to ohmically short the cathode-emitter region to the cathode-base region (shorted-emitter), and gating current is typically on the order of milliamps. Without the ohmic short, the SCR is extremely sensitive and may be triggered into conduction with a very small gate signal, typically on the order of microamps. This latter non-shorted-emitter design is subject to the above-noted problems with nuisance tripping.
The shorted-emitter design minimizes unwanted nuisance firing, but at the expense of gate sensitivity. Reduced gate sensitivity is not a significant obstacle if it is easy to supply, for example, milliamps instead of microamps to the gate.
Semiconductor switches which are activated solely by radiant energy are usually of the non-shorted-emitter design because they cannot sacrifice gate sensitivity. In the case of a light activated SCR, for example, it is not feasible to concentrate enough light energy on the SCR to cause turn-on when it has a shorted-emitter. Light activated SCR's are thus of the non-shorted-emitter design, and hence are sensitive to dv/dt, temperature, etc.
Continuing with the above example, numerous circuits have been devised to eliminate dv/dt and temperature problems in optical triggering applications. One approach is to use a shorted-emitter SCR, which may or may not be of the light activated type, and then insert a light activated switch in the gate circuit of the SCR which is in turn connected to the main load current line. Since the SCR has a shorted-emitter, a high amount of gate current is necessary to cause conduction. This gate current is supplied from the line terminal through the light activated switch to the gate terminal of the SCR, in response to impinging light.
A drawback of this latter type of arrangement is that two sources of energy are necessary for triggering, namely light energy and electrical energy. A second energy source, in addition to the light or other radiant energy, is needed to initiate conduction.
Another approach is to use a highly sensitive (eg. non-shorted-emitter) light activated SCR, and insert a bypass switch between gate and cathode which switch is in turn connected to the main load current line. Since the SCR is normally highly sensitive, it is subject to dv/dt induced nuisane tripping. The SCR is de-sensitized when the bypass switch is activated by line current.
A drawback of this last approach, as before, is that two sources of energy are necessary. Light energy impinging on the SCR is used to initiate conduction, and a second energy (eg. current from the line) is necessary to de-sensitize the device for dv/dt immunity purposes.